DIAGNOSTIC MEDICAL IMAGING 1st Part --Introduction. Ing. Tommaso Rossi

Transcription

1 DIAGNOSTIC MEDICAL IMAGING 1st Part --Introduction Ing. Tommaso Rossi Tommaso Rossi - Modulo di SEGNALI, a.a. 2013/2014

2 Overview 2 How we can look on the inside of human body? Invasive techniques: surgery, endoscope, etc. can cause damage or trauma to the body offer direct optical viewing Non-invasive techniques: medical imaging some of these techniques are completely risk-free, for others there are risks associted with the radiation exposure allow us to see things not visible to the naked eye

3 Brief History 3 The first published medical image was a radiograph Of Wilhelm Conrad Roentgen wife s hand (1895). Using a Crookes tube Roentgen discovered a new kind of rays, x-rays (wavelength between 10 nm and 10 pm), that could expose film even when optically sheilded. Few months later the first clinical use of x-rays occurred. Later the medical use of x-rays became common. Nuclear medicine arose from the discovery of radioactivity by Antoine Henri Becquerrel in The initial idea of using radioactive tracers to study human physiology was introduced by George de Hevesy in The modern scintillation camera was developed in 1952.

4 Brief History 4 The first interaction of acoustic waves with media was first described by Lord John Rayleigh at the end of Modern Ultrasound medical imaging was developed after the II World War due to the development of Navy sonar technology. Magnatic resonance imaging arises form the Nuclear magnetic resonance phenomenon, discovered by Felix Bloch and Edward Purcell that received the Nobel Prize in In 1971 the use of this phenomenon in medical imaging was suggested by Raymond Damadian and this concept was developed by Paul Lauterbur (who won the Nobel Prize in Medicine in 2003) in 1973.

5 Signals 5 Physical signals studied in medical imaging arise from different processes use of ionizing radiation a) Projection radiography and Computed Tomography scanning b) Nuclear medicine transmission of photons (x-rays) through the human body emission of photons (gamma rays) from radiotracers in the body use of electromagn. energy c) Magnetic resonance precession of spin systems in a large magnetic field d) Ultrasound imaging reflection of ultrasonic waves within the body use of sound waves

6 Projection Radiography 6 Projection of a 3D object or signal into a 2D image. The signal generator is a x-ray a tube able to create a x-ray pulse in a uniform con beam. The pulse, passing through the body, is attenuated by tissues. The signal intensity profile becomes non uniform and shadows are created by dense objects (i.e.: bones). The x-ray signal intensity profile is revealed through a scintillator that converts the signal to visible light, that is finally captured (on a film, a camera or a solidstate detector). Structures located at different deepts in the human body are superimposed on a 2D image

7 Computed Tomography CT uses x-rays not travelling in a 3D cone beam but collimated in a 2D fan beam. Shadows are created by tissues in a 2D cross-section and the signal intensity is detected by a large number of detectors. This measurement is called projection. Many projections are collected for different angular orientation of the tube signal generator (and detectors that rotate around the human body). Through these projections an image of the human body cross-section is computed (spatial resolution < 0.5 mm). 7 Different CT modalities: standard single-slice helical (whole body scan in less than a minute) multislice

8 Nuclear Medicine Images can be acquired only if appropriate radioactive substances (radiotracers) are introduced into the body. The image reflects the local concentration of a radiotracer within the body. Being this concentration tied to the physiological behaviour, this method is called functional imaging. e.g. radioactive iodine is used to study tyroid functions. Three main modalities: conventional radionuclide imaging or planar scintigraphy 8 single-photon emission computed tomography (SPECT) positron emission tomography (PET) emission computed tomography Planar scintigraphy and SPECT use radiotracres that are gamma emitters. PET uses radiotracers that emit positrons. SPECT and PET require tomographyc recnostruction while planar imaging forms images by projection.

9 Nuclear Medicine In contrast with projection radiography and computed tomography, the biological behaviour of a substance s biodistribution in the body is of interest in nuclear medicine. Each molecule of the substance is labeled with a radioactive atom. The ionizing radiation emitted when this atom undergoes radioactive decay is used to determine the location of the molecule within the body. a) Projection radiograph, image intensity reflects the varying absorption of transmitted x-rays through the bones (structural anatomical information) 9 b) Nuclear medicine bone scan, image intensity reflects the metabolic activity of the bones (metabolic information) a) b)

10 Nuclear Medicine 10 In nuclear medicine a 2D gamma ray detctor called Anger camera is Used (invented in 1952 by Hal Anger of the Donner Laboratory at the University of California). Anger camera is able to detect single rays. This procedure combines the effect of emission with effects of attenuation of rays due to body tissues. Images are 2D projections of 3D distribution of radiotracers plus Attenuation (spatial resolution 5-18 mm). Nuclear medicine images are based on the distribution of radiotracers, the interest is not in total intesity (as projection radiography and CT) but in the detected decay rate of the source, typically expressed as counts per time. Anger camera

11 Nuclear Medicine 11 In convetional radionuclide imaging and SPECT a radioactive atom s decay produces a single gamma ray which may be detected by Anger camera (a collimator is needed). In PET a radionuclide decay produces a positron, which annihilates with an electron producing two gamma rays flying off in opposite directions. PET scanner looks for coincident detections from opposing detectios in its ring, determining the line that passes through the site where the annihilation occured. SPECT scan that indicates the baseline blood reaching the brain PET-CT

12 Ultrasound Imaging 12 Ultrasound imaging uses electrical-to-acustical transducers to generate high frequency pulses (typically 1-10 MHz). These pulses travel through the body and reflect back to the transducer. time of return of the reflected pulses intensity of the reflected pulses gives information about location of the reflector gives information about the strength of the reflector Since ultrasound imaging systems have low image quality they are used to analyse the anatomy (realtime) They are very cheap and small

13 Ultrasound Imaging A-mode imaging: (or amplitude-mode) one-dimensional pulse waveform, used to generate detailed information about rapid or undetectable motion, i.e.: hearth valve motion B-mode imaging: ordinary cross-sectional anatomical imaging (2D image), created by a linear array of transducers scanning a plane through the body M-mode imaging: (or motion-mode) a succession of A-mode signals, each A-mode signal is a column in an image. Not an anatomical image but important for measuring of time-varying displacements Doppler imaging: uses the property of frequency or phase shift caused by moving objects to generate images that are colour coded by their motion 13 M-mode image mitral valve

14 Magnetic Resonance Magnetic resonance scanners use the property of nuclear magnetic Resonance (NMR) to create images In a strong magnetic field the nucleus of hydrogen tends to align itself with the field, creating a magnetization of the body. It is possible to excite a selected region of the body, moving away from the magnetic field direction groups of these little magnets. Once protons return back to be aligned with the field they experience a precession movement generating a radio-frequency wave that iscapturedbyanantenna. MR produces high-resolution high-contrast cross-sectional anatomic images and, like ultrasound imaging, is non-invasive. 14 Different kind of pulse sequences can be used to create different images, a clever combination of pulse sequences can be used to create dynamicseriesof images, which can be used to estimate blood flow (Functional Magnetic Resonance Imaging)

15 Magnetic Resonance 15 All nuclei have positive charges (they are composed by protons and neutrons). A nucleus with either an odd atomic number or an odd mass number has an angular momentum they have spin If the nuclei of the hydrogen atoms (¹H) are subjected to a strong magnetic field they tend to align with the field; being the number of hydrogen atoms into the human body very high, this tendency results in a magnetization of the body Φ N Nucleus angular momentum Microscopic magnetization of nucleus S

16 Magnetic Resonance 16 In normal conditions individual spins of ¹H nuclei have a random orientation, has results no macroscopic magnetic field is produced M = µ i = 0 µ is the magnetic moment vector B If a strong magnetic field, 0, is applied, the components of i vectors parallel to the field produce a macroscopic magnetic field 0 Nuclei spin precess around an axis along the direction of the field. This precession has a frequency, called Larmor frequency (rad/sec,proportional to ), of the order of MHz (radiofrequency) B 0 If a microscopic sample of nuclei is excited using a electromagnetic radiation having Larmor frequency, the radiation magnetic component interacts with nuclei magnetic moment µ A quantum of energy is absorbed changing the nuclei energy status The proton magnetisation vector is rotated by an arbitrary angle

17 Magnetic Resonance 17 When these energy transitions occur, nuclei are resonant with applied radiation When the external electromagnetic radiation ends, nuclei emit electromagnetic radiation at the same frequency in order to return to their previous energy state. The radio-frequency electromagnetic signature emitted by the nuclei can be sensed with an antenna and used for image reconstruction A magnetic resonance image has a medium spatial resolution but it is possible to obtain high tissues discrimination. The operator can choose in real-time to analyse different tissues characteristics Paramagnetic contrast-agents / tracers can be used to improve MR imaging (enhanced contrast and measurement of additional functions)

18 PACS System Picture Archiving and Communication System is a software and hardware system for medical images archiving, transimssion and visualization. 18 A PACS is composed by a file archive (able to manage data and images) and visual display units, able to represent images on an high resolution Monitor. Images/data shall not be modified, hence usually the archiving process is done using a legal archive. The new generation of PACS is able to process the images, i.e. creating 3D reconstructions. PACS is integrated with the RIS (Radiology Information System) that is the software for the management of the radiology ward.

19 DICOM Standard 19 Digital Imaging and COmmunication in Medicine Standard is a standard for the exchange of medical images in a digital format. It has been created to solve the problem of information sharing DICOM has been developed by National Electrical Manufacturers Association (NEMA) in conjunction with the American College of Radiology (ACR). The first version was released in 1985 DICOM is designed to ensure the interoperability of systems used to: Produce, Store, Display, Process, Transmit, Handle or Print medical images and derived structured documents as well as to manage related workflow.

20 DICOM Standard 20 DICOM is used in: radiology breast imaging cardiology radiotherapy oncology ophthalmology dentistry pathology surgery veterinary neurology pneumology DICOM is an industrial standard (not an ISO standard) In general the equipments are partially DICOM compliant DICOM standard includes both a file format definition and a network communication protocol; a large class of services can be provided The communication protocol is an application protocol that uses TCP/IP to communicate between systems.

21 DICOM Standard DICOM does not define new algorithms for image compression but a standard for data encapsulation. A DICOM image consists of a header and a content: the header is a long stream of textual information that specify the type of content (patient identification attributes, data on the type of exam, etc.) and other administrative info the content is the medical image data (it can be compressed or not) 21 Imaging modality Radiology Information System Workstation DICOM Network Other Networks Printer Digital Archive